The present invention relates to an organopolysiloxane composition curable by condensation reaction or, in particular, to an organopolysiloxane composition curable by condensation reaction with extended pot life.
The condensation reaction between organopolysiloxanes is a well known reaction and widely utilized in preparing various kinds of silicone products. The products by the condensation reaction are used as intermediate compounds for the preparation of various kinds of products with complicated structures or used per se as final objective products of value. For example, they are used as a mold release agent or a coating material on a release paper. When the intended use of these organopolysiloxane compositions is in the preparation of a release paper, they are put to use by adding a catalyst for accelerating the condensation reaction. The catalyst-added organopolysiloxane composition has desirably a sufficiently long pot life at room temperature. In other words, the velocity of the crosslink formation in the composition by the condensation reaction should be as low as possible at room temperature after admixing of the condensation catalyst. On the other hand, the curing velocity of the catalyst-added organopolysiloxane composition must be sufficiently large at an elevated temperature to give good productivity in the production process by use of the composition.
In recent years, there is known an organopolysiloxane composition curable by condensation reaction with improved curing velocity but such a type of the composition suffers from shortened pot life. The requirements for the pot life as long as possible and for the large curing velocity cannot be compatible with each other because an extended pot life is obtained only in the sacrifice of the curing velocity. This problem of undesirably short pot life is especially difficult to solve in an organopolysiloxane composition in which the condensation reaction takes place by the dehydrogenation reaction between silicon-bonded hydroxy groups and silicon-bonded hydrogen atoms among various types of the condensation reactions including dehydration condensation between silicon-bonded hydroxy groups, dealcoholation condensation between silicon-bonded hydroxy groups and silicon-bonded alkoxy groups and de(carboxylic acid) condensation between silicon-bonded hydroxy groups and silicon-bonded acyloxy groups and the like.
Therefore it has been a generally accepted practice that the extension of the pot life is achieved by the dilution of the organopolysiloxane composition having large curing velocity with large volume of an organic solvent. As a trend in recent years, however, the amount of an organic solvent to be added to such a composition is limited further and further from the standpoint of toxicity against human body, danger of fire or explosion and the problem of air pollution as well as from the standpoint of saving of natural resources. Thus it is eagerly desired to develop an organopolysiloxane composition having sufficiently extended pot life along with no influence on the curing velocity and on the properties of the composition after curing by use of a minimum volume of an organic solvent.
Besides the organopolysiloxane compositions curable by condensation reaction, on the other hand, there is known another type of organopolysiloxane compositions curable by addition reaction. For this type of organopolysiloxane compositions, several kinds of effective reaction retarders are proposed such as acetylenic compounds as a result of investigations and they are widely utilized industrially.
Unfortunately, no reaction retarders are known for the organopolysiloxane compositions curable by condensation reaction with exception of acetic acid and the like with only limited effects (see Japanese Patent Publication SHO 38-13913) necessitating to recur to the use of large volumes of organic solvents to dilute the composition. In addition, the method relying on acetic and the like is inapplicable to the organopolysiloxane compositions used in a high solid concentration or as solvent-free and it is also inapplicable to the organopolysiloxane compositions used in the form of an aqueous emulsion since such an additive is sometimes harmful to the stability of the aqueous emulsion.
In addition, the curable organopolysiloxane compositions of the above described type have a problem that, when used as a coating agent on release papers, the coating and heat-curing steps of the composition must follow as soon as possible the admixing of the curing catalyst into the composition because a delay in the coating and heat-curing sometimes leads to undesirable increase in the peeling resistance of the finished release paper products to such an extent as inapplicable to practical use.
In short, the conventional reaction retarders described above have very little effectiveness in retarding the condensation reaction and are hardly applicable to the organopolysiloxane compositions of high-solid type or aqueous emulsion type.